The invention relates to a relay valve for a vehicle, including a parking brake piston that is displaceable axially in a housing and an axially displaceable cuff. A first compressed air chamber for receiving compressed air from a control line is formed between the parking brake piston and the housing. A first spring element also being provided, which loads the cuff axially with a preloading force. And, a second spring element is provided, which loads the parking brake piston axially with a further preloading force. The invention also relates to a method for operating such a relay valve.
Such a relay valve is primarily used in parking brake devices for commercial vehicles, in order to implement a secure anti-compounding function.
Parking brakes—also called emergency brakes—of commercial vehicles, including trailers but also rail vehicles, are currently regularly equipped with spring accumulator brake cylinders which, in a released position, apply compressed air to a spring compression chamber and as a result keep the spring loaded, while for the parking brake function, the spring compression chamber is vented, i.e. connected to atmospheric pressure, so that the brake cylinder generates a braking force under the action of the spring (cf. Bosch, Motor Vehicle Technical Pocketbook, 22nd edition, Dusseldorf, 1995, p. 648).
In current braking systems for commercial vehicles, relay valves in combination with spring accumulator brake cylinders are installed for the parking brake function. When the vehicle is parked, the control line of the relay valve is vented, so that the spring accumulators are also vented. Strong springs in the spring accumulators press the brake pads against the brake disc. These spring forces have to be absorbed by the brake calipers, which leads to high stresses in the brake calipers.
If, in the parked state with no anti-compounding function, the service brake were also to be actuated, the brake pads would additionally also be pressed pneumatically against the brake disc. These forces would likewise have to be absorbed by the brake calipers, so that, as a result of this superimposition, overloading of the brake calipers would occur.
This overloading of the brake calipers is prevented by the anti-compounding function. Overloading when the parking brake is engaged and with simultaneous actuation of the service brake is no longer possible, since the forces on the brake caliper from the parking brake are reduced to the same extent as the forces on the brake caliper from the service brake rise. As a rule, a changeover valve (select-high valve) is incorporated in the control line of the relay valve for this purpose. When the control line of the relay valve is vented (parked state), the brake pressures can reach the control line of the relay valve via the select-high valve. The resultant pressure rise in the spring accumulators reduces the braking forces of the parking brake.
EP 2 240 352 B1 discloses a parking brake device for motor vehicles having a spring accumulator brake cylinder that can be activated by a relay valve. The relay valve can be activated via a safety valve formed as a 3/2-way valve, the output of which can optionally be connected to one of two inputs. The inputs of the safety valve are connectable optionally to a pressure medium source or atmospheric pressure via a first or second solenoid valve. A select-low valve is connected to the one input and the output of the safety valve, and an output of the select-low valve is connected to a control input of the relay valve. An anti-compounding function is achieved in that, between the select-low valve and the pneumatic control input of the relay valve, a select-high valve is also connected, one input of which is fed with the output from the select-low valve and the other input of which is fed with the pressure of the service brake. The select-high valve selects the higher of the two pressures and feeds the same to the control input of the relay valve. It is therefore ensured that the control input of the relay valve is only ever loaded with an unambiguously defined pressure and that the service brake has priority over the parking brake.
Furthermore, EP 2 407 355 B1 discloses a relay valve having a control piston and a cuff, both being displaceable in an axial direction. A first resilient element is provided, which loads the cuff in the axial direction with a preloading force, and also a second resilient element, which loads the control piston in the axial direction with a further preloading force. A control line of the electrically operable emergency braking system in which the relay valve is arranged is connected via a relay control line and a changeover valve to the relay control input of the relay valve. Also connected to the changeover valve is a service brake line. The service brake pressure is led to the relay valve via the changeover valve. The latter discharges pressure into the spring accumulators in accordance with the activation, which pressure works against the spring force and service brake pressure and, for the most part, compensates for the latter. Overloading of the components can thus be avoided. The first resilient element is used to offset the characteristic curve. The offset of the characteristic curve is required by a control valve device, what is known as a PCV valve. Starting from, a certain control pressure, the latter switches from the driving position into the parking position. An auxiliary braking action is achieved in that, in the driving position, the control pressure is reduced. In order for the control valve device to remain in the driving position and for the lowest possible pressure to be reached on the output from the relay valve, there exists an offset of the relay valve characteristic curve. This offset has the disadvantage that, with an anti-compounding function via a select-high valve, the action of relieving the brake calipers by filling the spring accumulators takes place too slowly. As a result, despite the anti-compounding function, overloading on the brake caliper arises in certain situations.
The object of the present invention is to further develop a relay valve in order to implement an improved anti-compounding function.
This and other objects are achieved according to the invention by an axially displaceable service brake piston arranged axially between the parking brake piston and the cuff, said service brake piston coming into contact at least radially against the parking brake piston and radially against the housing in a sealing manner, in order to form a second compressed air chamber. As a result, the disadvantage of overloading the relay valve having a piston is avoided. The air pressure from a control line connected to a control connection acts on the service brake piston without offset, on account of the resilient element. As a result, an anti-compounding characteristic curve is not displaced, so that the anti-compounding function is achieved more rapidly. Furthermore, a valve unit is dispensed with, in particular the select-high valve. As a result, both the overall space requirement and the outlay on assembly are reduced.
As a result of the load-relieving surface that is produced between the parking brake piston and the service brake piston, the anti-compounding characteristic curve becomes flatter. This has the advantage that the compensation of the forces on the brake caliper runs more linearly, so that a “roll-back effect” is avoided. The roll-back effect appears when, for example, a truck is standing on a hill and the anti-compounding function acts during parking. If a certain force is undershot during the compensation operation of the forces on the brake caliper, the truck rolls.
The housing can, in particular, include a plurality of partial housings, but preferably two, which are connected to one another. The service brake piston together with the parking brake piston forms a dual piston. Advantageously, the parking brake piston and the service brake piston have a circumferential groove on the outer circumferential surface thereof, in each of which there is arranged a sealing element, in particular an O-ring, which seals off the pressure chambers pneumatically. Furthermore, a further sealing element, in particular an O-ring, is arranged radially between the parking brake piston and the service brake piston.
It is proposed that it be possible for the second compressed air chamber to be fed directly with compressed air from a service brake line, which is arranged on a service brake. In other words, no further valves or elements similarly affecting the pressure, such as a select-high valve or a changeover valve, for example, are located in the service brake line between the relay valve and the service brake. Consequently, the anti-compounding function is implemented directly in the relay valve by the functionality of the dual piston.
Preferably, a third compressed air chamber for setting the air pressure in an output line connected via a venting connection is formed between the service brake piston, the housing and the cuff. The air pressure in the third compressed air chamber thus acts on an end face of the service brake piston, counter to the air pressure in the second compressed air chamber. Furthermore, however, the parking brake piston also acts axially on the service piston.
Furthermore, it is preferred for it to be possible for a pneumatic inlet of compressed air from a supply line, connected via a supply connection, for venting the third compressed air chamber to be formed axially between a valve seat on the housing side and the cuff. In other words, the valve seat with which the cuff comes into contact on account of the preloading force of the spring element is formed on the housing. If the cuff is displaced axially counter to the preloading force of the spring element, the inlet opens, so that compressed air from the supply line for venting the third chamber flows in.
The invention includes the technical teaching that a pneumatic outlet for venting the third chamber can be formed axially between a valve seat formed on the service brake piston and the cuff. If the third chamber is vented, the spring accumulator brake cylinders are also vented, so that the brake acts. The pneumatic outlet is closed by the service brake piston resting axially on the cuff.
Furthermore, a method for operating a relay valve is provided, which comprises at least five operating modes, specifically the operation of the relay valve:                when the vehicle is parked,        in the event of the service brake being actuated during the parked state,        when the vehicle is being driven,        when reducing the air pressure present on the output line, and        when increasing the air pressure present on the output line.        
When the vehicle is parked, the control line, the service brake line and the output line are depressurized, the cuff coming axially into contact with the housing-side valve seat and the service brake piston being spaced axially apart from the cuff, so that the third compressed air chamber is vented via the opened outlet.
In the event that the service brake is actuated while the vehicle is parked, the service brake piston comes axially into contact with the cuff, the cuff being displaced axially counter to the preloading force of the first spring element, so that the inlet is opened and the outlet is closed.
When the vehicle is being driven, the control line, the supply line and the output line are loaded with compressed air, the service brake piston coming axially into contact with the cuff, so that the inlet and the outlet are closed.
When reducing the air pressure present on the output line, the air pressure in the first compressed air chamber is reduced in a controlled manner, the service brake piston lifting axially off the cuff, so that the outlet is opened and the third compressed air chamber is vented in a controlled manner.
When raising the air pressure present on the output line, the air pressure in the first compressed air chamber is increased in a controlled manner, the service brake piston displacing the cuff axially counter to the preloading force of the first spring element, so that the inlet is opened and the third compressed air chamber is vented in a controlled manner.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.